The present invention relates to a shift member and more particularly to a heat-resistant shift member, and also relates to a method for manufacturing the same.
Here the term "heat-resistant shift member" used throughout the present Specification and claims should be interpreted to designate an element which can carry a load with a low coefficient of friction even under temperatures in which the application of generally used lubricant oils is difficult and additionally has a sealing function, i.e. as a sealing element to be used at elevated temperatures, e.g. a bearing bush, a washer, a sliding board, a contact type packing or the like.
Hitherto, as a shift member of this kind a metallic member made of stainless steel or copper alloy within which is embedded pellets of a solid lubricant, a non-metallic material made of graphite, ceramics, etc., a compound material such as the so-called "cermet" and heat-resistant plastic material such as polytetrafluoroethylene resin (PTFE), polyimid, etc. have been publicly known.
However, although these materials are all excellent in heat resistance, they have problems in friction and wear under dry friction conditions and difficulties in mechanical strength, in particular shock impact resistance, not necessarily having good conformability with the counter member, and cannot fulfill those functions sufficiently in the case of minute sliding.
In order to solve these difficulties or problems, as disclosed in e.g. U.S. Pat. No. 1,137,373, Japanese Patent Publication No. 23966/1969 or its corresponding U.S. Pat. No. 3,404,061, etc. a shift member has already been developed wherein expanded graphite obtainable by a special treatment of graphite is shaped in association with a reinforcing member. Although these publicly known shift members have good heat resistance revealing superiority in conformability with the counter member and have improved shock impact resistance compared with regular graphite, the coefficient of friction is rather somewhat higher than that of regular graphite and they have a defect in that they often generate abnormal frictional sounds at the time of shifting under dry friction conditions.
Further a shift member which is produced by similarly shaping a heat-resistant material such as mica, asbestos, etc. together with reinforcing materials has also been publicly known, but it has problems similar to those in the above described shift members.
The reasons why therefor, are considered to reside in the fact that these shift members have large differences in friction coefficients between their static and dynamic friction, and they have a certain extent of flexibility, and also it is considered that the shapes and the natural frequencies of the various components constituting the slip system have an effect on those phenomenon.
In order to solve the above problems the present inventors have already proposed an invention concerning a heat-resistant shift member as Japanese Patent Application No. 140987/1982, filed on Aug. 16, 1982, or its corresponding U.S. patent application Ser. No. 522009 (filed on Aug. 11, 1983) (hereinafter referred to as "specific prior art").
Here this specific prior art will be briefly explained below.
That is, the heat-resistant shift member in accordance with this specific prior art is characterized in that a shift member base material having generally the desired configuration is prepared by shaping one or more of heat-resistant materials selected from the group comprised of expanded graphite, mica, asbestos, etc. together with a reinforcing material of stainless steel mesh so as to have the heat-resistant materials and the reinforcing material integrally confined with each other, and that a shifting surface layer member is prepared which is manufactured such that a heat-resistant sheet comprised of paper, non-woven cloth, woven cloth composed of one or more filaments made of asbestos, carbon (excluding expanded graphite), glass, etc. is covered with powdered solid lublicating compounds, and the heat-resistant sheet thus covered with the lubricating compound is reinforced by putting it on a metal mesh made of fine copper alloy wires, and that the surface of the shift member base material is integrally covered with the shifting surface layer member under compression such that both the metal meshes are entangled with each other.
However, it has been confirmed as a result of being put into practice that although this specific prior art revealed such advantageous properties that the shifting surface layer member generated little abnormal frictional sounds when it shifted under friction relative to the counter member, and showed good wear resistance, new problems were also found such as at the initial stage of friction, in other words, during the period until the powered solid lubricating compound of the shifting surface layer member is transferred on to the surface of the counter member to form thereon a solid lubricating compound film the shifting surface exhibited high frictional resistance (high coefficient of friction), thereby generating abnormal friction sounds and also that at elevated temperatures there was oxidation of the metal mesh made of fine copper alloy wires, which reinforced the shifting surface layer member, causing a siezure of the metal mesh with the counter member.
Among the above problems, a reduction of the high shifting frictional resistance and the generation of abnormal frictional sounds at the initial state of friction can be sought by increasing the compounding amount of polytetrafluoroethylene resin (PTFE) in the powdered solid lubricating compounds of the specific prior art so that its ratio exposed to the shifting surface layer member is increased.
However, with this high compounding amount of PTFE, the PTFE becomes interposed between the interface of the other solid lubricating compounds, which causes the solid lubricating compounds to not be combined with each other, and results in creating various problems originating from the heat expansion and softening of the PTFE particularly at elevated temperature ranges.
That is, at elevated temperature ranges, due to the heat expansion and softening of the PTFE existing between the interface of the powdered solid lubricating compounds the combining force of the powdered solid lubricating compounds with each other is weakened, generating protrusions on the shifting surface layer so that the strength of the shifting surface layer member is considerably decreased. Therefore, the weakening of the combining force of the powdered solid lubricating compounds and the protrusions of the shifting surface layer member cause the softened PTFE to flow with the powdered solid lubricating compounds, the shifting surface layer member to fall off from the surface of the base material, and moving towards a frictional shifting between the heat-resistant material (expanded graphite, etc.) of the shift member base material and the counter member, causing generation of abnormal sounds, etc.